Humans as the ideal space mercenaries -- I recall something like that in the book Killbird by Zach Hughes. But I suspect that idea is not unique to that one author.
There's a tumblr post about this. Kinda. Less 'mercenary', more... super soldiers.
(it's rather long, but it's worth the read I think)
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Hate me if you want. But that's your issue to fix, not mine.